The present invention relates to a thin film sensor having a thin film that includes a pair of insulating films sandwiching a metallic resistor film, a method of manufacturing such a thin film sensor, a flow sensor having the said thin film, and a method of manufacturing such a flow sensor.
Flow sensors, infrared light sensors, and gas sensors are examples of thin film sensors having a resistor film (metallic resistor film), made of metal like platinum, on a micro bridge or a membrane and relying on temperature-dependent changes in resistance in the metallic resistor film for sensing.
In general, such sensors include a lower insulating film, metallic resistor film, and upper insulating film, which are deposited in layers on a substrate to form a thin film. Usually, an adhesion layer consisting of metallic oxide is placed between the lower insulating film and the metallic resistor film to improve adhesion between the metallic resistor film and the insulating film (SiO2 or SiN) and to prevent the metallic resistor film from peeling.
In order to take advantage of temperature-dependent changes in the resistance of the metallic resistor film, the metallic resistor film, along with the adhesion layer underneath, is patterned into serpentine lines for active parts. Wiring used for sensing is made of a patterned multilayer film consisting of the metallic resistor film and the adhesion layer.
The inventors of the present invention have discovered that the thermal coefficient of resistance (TCR) for the serpentine lines decreases as a result of a thermal treatment that takes place after the lower insulating film and the serpentine lines (the adhesion layer made of metallic oxide and the metallic resistor film) are formed on the substrate for a conventional thin film sensor.
Consider an example of a flow sensor in regard to the problem of decreased TCR in the serpentine lines. A conventional thin film flow sensor is shown in FIG. 11 and FIG. 12. FIG. 12 shows patterns for active parts 3, 4 and 5 in the flow sensor.
A thin film (membrane) 10 is formed on a substrate 1, which has a cavity 1a, and covers the cavity 1a. A heater 5 and a temperature sensor 4, which are two of the active parts, are formed on the thin film 10 over the cavity 1a. A flow thermometer 3, which is also an active part, is formed on the thin film 10 on the substrate 1 but not above the cavity 1a. 
The active parts 3, 4, 5 are formed into striped patterns. The flow thermometer 3, temperature sensor 4, and heater 5 are placed in that order along the path indicated by the arrow in FIG. 11, which also shows the direction of fluid flow.
In this flow sensor, the heater 5 is activated in such a way that the temperature of the heater 5 is above the fluid temperature, which detected by the flow thermometer 3, by a prescribed level. When the flow is in the direction of the arrow in FIG. 11, the temperature sensor 4 loses heat, and the temperature of the temperature sensor 4 falls. When the flow is in the opposite direction from the arrow, the temperature sensor 4 receives heat, and its temperature increases. It is possible, therefore, to detect the direction of the flow and the rate of flow from the temperature difference between the temperature sensor 4 and the flow thermometer 3. Temperature is measured (detected) from changes in resistance in the metallic lines that include the flow thermometer 3 and the temperature sensor 4.
FIGS. 13A to 13D and FIGS. 14A to 14C show a generally used manufacturing method for such a flow sensor. FIGS. 13A to 13D and FIGS. 14A to 14C show the steps for manufacturing the flow sensor of FIG. 14C. FIG. 14C is a simplified cross sectional view taken along line 14xe2x80x9414 in FIG. 11.
Firstly, as shown in FIG. 13A, a silicon nitride film 21 is deposited by a low pressure CVD method on a surface of a silicon substrate 1. Then, a silicon oxide film 22 is deposited by a plasma CVD method on top of this film to form a lower insulating film 2 that consists of the two layers 21 and 22 (lower insulating film formation step). Next, the film properties of the lower insulating film 2 (stability under stress and strength) are improved by annealing in a furnace with a nitrogen atmosphere (lower insulating film anneal step).
Next, as shown in FIG. 13B, a multilayer film 3c, consisting of a titanium film 3a, an adhesion layer, a platinum film 3b, and a metallic resistor film, stacked in that order, is deposited on the lower insulating film 2 by vapor phase deposition or sputtering. Next, the multilayer film 3c is annealed in a furnace with a nitrogen atmosphere to improve the film characteristics and TCR. With the annealing step, the titanium film 3a turns into a metallic oxide that makes up the adhesion layer (titanium oxide in this example). Next, a resist film 4a, which has patterns corresponding to the active parts 3, 4, 5, is formed on the multilayer film 3c. 
As shown next in FIG. 13C, using the resist film 4a as a mask, the multilayer film 3c is etched by, for example, ion milling to form the active parts 3, 4, 5. The steps described so far include a step for depositing the multilayer film 3c, a step for annealing and patterning, a step for forming the adhesion layer 3a consisting of metallic oxide on top of the lower insulating film 2 (adhesion layer formation step), a step for forming a resistor film 3b, consisting of a metal, on top of the adhesion layer (resistor film formation step), and a step for forming the active parts 3, 4, 5 (active part formation step).
Then, a silicon oxide film 61 is deposited by a method such as plasma CVD to cover the active parts 3, 4, 5 on the lower insulating film 2. After an annealing step, which is performed in a furnace with a nitrogen atmosphere, a silicon nitride film 62 is deposited by a method such as low pressure CVD to form an upper insulating film 6, which consists of the two layers 61 and 62 (upper insulating film formation step).
Next, as shown in FIG. 13D, openings 7a are formed in the upper insulating film 6 for forming pads 7 (shown in FIG. 11) for the active parts 3, 4, 5. As shown in FIG. 14A, the pads 7, which are made of, for example, gold (Au), are formed by methods such as vapor phase deposition, sputtering, photolithography, and etching.
As shown in FIG. 14B, a silicon oxide film 8 is formed by, for example, a method such as plasma CVD on the other surface of the silicon substrate 1. This film 8 is then etched to form an opening 8a, which corresponds to the cavity 1a mentioned earlier. Next, as shown in FIG. 14C, the silicon oxide film 8 is used as a mask for anisotropic etching on the silicon substrate 1, so that the cavity 1a is formed and the silicon nitride film 21 on the other side of the silicon substrate 1 is exposed (cavity formation step).
Using the steps described above, it is possible to manufacture the flow sensor shown in FIG. 11. As shown in FIG. 14C, the structure of the flow sensor includes the thin film structure formed over the cavity 1a on the substrate 1, which includes the cavity 1a, with this thin film structure 10 including the lower insulating film 2, adhesion layer 3a consisting of a metallic oxide, metallic resistor film 3b, and upper insulating film 6, which are stacked on top of each other on the substrate 1. Active parts 3, 4, 5 are formed by patterning the multilayer film 3a, consisting of the adhesion layer 3a and the resistor film 3b, into prescribed patterns.
The method of manufacturing described above requires several high temperature process steps after the active parts 3, 4, 5 are formed, including the high temperature deposition step for the upper insulating film 2 and the annealing step for improving the stability and TCR characteristics of the active parts 3, 4, 5 and for improving the insulating films 2 and 6, in terms of strength and stability under stress.
In this manufacturing process, the TCR characteristics of the active parts 3, 4, 5 degrade with the high temperature (approximately 800xc2x0 C.) used for the low pressure CVD method of depositing the silicon nitride film 62 for the upper insulating film 6 or during the annealing step for the PE-SiN film 62, which follows the deposition of the silicon nitride film 62 by a low temperature plasma CVD process.
The cause of this degradation is thought to be the mechanism illustrated in FIG. 15. Reduction gas agents, such as hydrogen (symbolized by the letter H in FIG. 15), are generated by a source gas during deposition of the silicon nitride film 62 or by the nitride film 62 as this film gets annealed. The active parts 3, 4, 5 are exposed to this reduction gas atmosphere at a high temperature.
As a result of this exposure, the metallic oxide film, which makes up the adhesion layer in the active parts 3, 4, 5 (for example, TiO2), is reduced back to a metal. The reduced metal atoms diffuse into the metallic resistor film (platinum film). As a result, the TCR for the active parts 3, 4, 5 decreases. Therefore, it is necessary to prevent the metallic oxide in the adhesion layer from being reduced.
More specifically, the reaction that takes place during the low pressure CVD process for the silicon nitride film 62 is SiH2Cl2+NH3xe2x86x92Si3N4+H2+HCl. Hydrogen is generated, and a large amount of hydrogen is incorporated into the silicon oxide films 22 and 61. On the other hand, when the PE-SiN film, which is formed at a low temperature, is annealed at a high temperature, the PE-SiN film generates hydrogen, which is incorporated into the silicon oxide films 22 and 61.
According to this theorized mechanism, it would be impossible to completely avoid a decrease in the TCR for the active parts 3, 4, 5. Because the active parts 3, 4, 5 are patterned, the reduction gas agent will be incorporated into the silicon oxide films 22 and 61 at areas A (shown in FIG. 15) surrounding the active parts 3, 4, 5, where the active parts 3, 4, 5 are not present, will diffuse through the oxide films, and arrive at the active parts 3, 4, 5.
The metallic oxide film, which makes up the adhesion layer, is reduced to an even higher degree because of the additional reduction gas agent from the areas A. As a result, the TCR values for the active parts 3, 4, 5 decrease significantly.
In a device like a flow sensor, in which the active parts 3, 4, 5 are patterned into several different shapes, the sizes of the areas A (surface area) surrounding the various active parts will differ from one another.
Because of the different roles played by the active parts 3, 4, 5, the serpentine line widths also vary. For example in a flow sensor, the resistance values required for the heater 5, temperature sensor 4 and flow thermometer 3 are determined by the driver circuits. The resistor value for the heater 5 is around several hundred ohms, while the resistor values for the temperature sensor 4 and flow thermometer 3 are several kilo-Ohms.
Because the sizes of the surrounding areas A and the line widths of the active parts vary, different amounts of reduction gas agent are incorporated into different active parts. For this reason, the degree by which the TCR decreases would also be inconsistent among the various active parts. This problem can complicate the design of the driver circuits and should be avoided.
Therefore, thin film sensors in general, including the flow sensor described above, are susceptible to the problem of decreased TCR in the active parts, when these thin film sensors have active parts formed by a multilayer film that includes a lower electrode insulating film on a substrate, an adhesion layer consisting of metallic oxide, a metallic resistor film, and an upper insulating film, which are deposited on top of each other and patterned, because a reduction gas agent is generated during the deposition of the upper insulating film after the active parts are formed and during annealing.
An objective of the present invention, therefore, is to address the problem described above and to minimize decreases in the TCR values in the active parts in a thin film sensor.
To achieve this objective, the invention is essentially a thin film sensor including a lower insulating film; an adhesion layer, which includes a metallic oxide material; a resistor film, which includes a metal, wherein the adhesion layer and the resistor film form a multilayer film; and an upper insulating film. The lower insulating film, the adhesion layer and the resistor film are stacked on a substrate. The thin film sensor further includes an active part, which is formed by patterning the multilayer film into a predetermined shape, and a dummy film layer, which is made of the same material that the active part is made of. The film layer is located in close proximity to the active part. The dummy film layer protects the active part from the effects of reduction gas.
In another aspect, the invention is a method of manufacturing a thin film sensor including: forming a lower insulating film; forming an adhesion layer, which includes a metallic oxide material, on the lower insulating film; forming a resistor film, which includes a metal, on the adhesion layer, wherein the resistor film and the adhesion layer form a multilayer film. The method further includes forming an active part by patterning the multilayer film into a predetermined shape; forming a dummy film layer simultaneously with the forming of the active part; forming an upper insulating film to cover the active part by forming a silicon nitride film by a plasma CVD method; and annealing the silicon nitride film.
In another aspect, the invention is a method of manufacturing a thin film sensor comprising: forming a lower insulating film; forming an adhesion layer, which is comprised of a metallic oxide material, on the lower insulating film; forming a resistor film, which is comprised of a metal, on the adhesion layer, wherein the resistor film and the adhesion layer form a multilayer film; forming an active part by patterning the multilayer film into a predetermined shape; and forming an upper insulating film to cover the active parts by forming a silicon nitride film by a plasma CVD method; forming through holes to pass through the silicon nitride film; and annealing the silicon nitride film. Reduction gas agent is released easily through the through holes formed in this method.